The present invention relates to power steering systems, and more particularly to such steering systems of the type which are typically referred to as being of the xe2x80x9csteer-by wirexe2x80x9d type.
As will be understood by those skilled in the art, a typical steer-by-wire system is to be distinguished from the known, prior art vehicle steering systems of the following types:
(1) mechanical steering systems, in which there is a mechanical link between the steering wheel and the steered wheels and the entire steering torque to the steered wheels is merely the result of steering input effort provided by the vehicle operator;
(2) mechanical steering with hydraulic power assist, in which there is a mechanical link between the steering wheel and the steered wheels, but the operator input is assisted by hydrostatic fluid pressure acting on, for example, opposite ends of a rack and pinion device; and
(3) a fully hydrostatic power steering system, in which there is no mechanical link between the steering wheel and the steered wheels, and all steering torque is the result of hydrostatic fluid pressure communicated to a steering actuator from a steering control unit (SCU), the input to which is the manual steering effort of the operator.
A typical steer-by-wire system could be a type of hydrostatic power steering system, but could also be a fully electric steering system, as will be explained in greater detail subsequently. In the typical steer-by-wire system, the rotation of the steering wheel by the vehicle operator is sensed, and results in the generation of a steering command signal which is transmitted as one of the inputs to an electrical logic device, such as the vehicle microprocessor. Another input to the microprocessor is the instantaneous steered wheel position, typically generated by a sensor associated with the steering cylinder. In response to the various input signals received, the microprocessor transmits a control signal to a proportional steering valve, which controls the flow of fluid from a steering pump to the steering cylinder. Therefore, the only xe2x80x9cconnectionxe2x80x9d between the vehicle operator and the steering valve is the electrical command signal (wire) from the microprocessor, and hence, the name xe2x80x9csteer-by-wirexe2x80x9d.
Steer-by-wire systems are becoming increasingly popular on a variety of vehicle applications. In the conventional fully hydrostatic power steering system, there is a steering input device (such as the SCU), which receives its input from the steering wheel by means of the steering column, and therefore, the SCU is typically disposed in, or immediately adjacent the operator compartment. In such a hydrostatic steering system, all of the flow within the steering system passes through the SCU, and therefore, there is a large flow of pressurized fluid in close proximity to the operator compartment. As a result, there is likely to be the noise and vibration normally associated with the flow of pressurized fluid, readily discernible by the operator. In the case of a steer-by-wire system, the steering input device disposed in or near the operator compartment does not have the steering system fluid flow passing through it. Only the proportional steering valve has substantial fluid flow passing through it, but the proportional steering valve is typically mounted somewhere near the steering cylinder, well away from the operator compartment. Therefore, the device which constitutes the major source of noise and vibration in a hydrostatic power steering system is eliminated in a steer-by-wire system, and replaced by a valve which does not have to be located in or near the operator compartment.
One of the problems associated with the typical, prior art steer-by-wire steering system is a relatively poor xe2x80x9ctorque feelxe2x80x9d. In many such systems, the input portion of the system merely comprises a steering wheel and steering column, and the associated sensors needed to sense steering wheel position, rate of rotation, etc. As a result, when the vehicle operator turns the steering wheel to accomplish a steering maneuver, there is very little torque feel of the type the operator would normally experience when rotating the steering wheel in any of the known mechanical/hydrostatic systems described above.
In many vehicle applications, it would be desirable for the steer-by-wire system to have not only an appropriate level of torque feel, but also, the ability for the system to vary the torque feel to correspond to changes in the various steering system parameters. For example, as one system design option, the steer-by-wire system could be made to have a feel which is very similar to what the operator would normally experience in driving an automobile, in which the reaction torque could be made to decrease as the vehicle speed increases. Alternatively, the steer-by-wire system could be made to have a feel similar to conventional hydrostatic power steering in which the torque feel could be made to increase as the vehicle operator would increase the rate of rotation of the steering wheel.
Finally, it would be desirable for the torque feel to greatly increase, to the point of effectively preventing further rotation of the steering wheel, whenever the steered wheels approach the xe2x80x9cstopsxe2x80x9d, i.e., when the steered wheels reach their maximum steering angle. Unfortunately, in some prior art systems, the attempt to increase the reaction torque as the operator steers into a stop has merely hampered the subsequent steering operation in the opposite direction, away from the stop. By way of example only, in some systems which have been proposed, the means which provides the increased torque feel just before reaching the steering stops would still be engaged or xe2x80x9cappliedxe2x80x9d as the operator would attempt to steer away from the stop, i.e., the torque feel would be as great steering away from the stops as it was approaching the stops, which is clearly undesirable.
Accordingly, it is an object of the present invention to provide an improved steer-by-wire vehicle power steering system which overcomes the disadvantages of the prior art systems, and which is capable of providing the vehicle operator with improved torque feel.
It is a more specific object of the present invention to provide such an improved steer-by-wire system which is also capable of varying the torque feel experienced by the operator in response to variations in vehicle and steering system operating parameters.
It is a further object of the present invention to provide such an improved steer-by-wire system which accomplishes the above-stated objects, and which includes the ability to provide the operator with substantially increased torque feel when the system reaches the end of steering stops, while maintaining the ability to steer easily away from the stop.
The above and other objects of the invention are accomplished by the provision of an improved steer-by-wire vehicle steering system including a steered wheel actuator assembly operable to provide steering movement to a pair of steered wheels in response to changes in an input signal. A controller is operable to generate the input signal for transmittal to the steered wheel actuator assembly, and to generate a feedback signal. A steering actuator assembly includes a steering input device adapted to receive a steering input motion from a vehicle operator, a steering reaction device adapted to be driven by the steering input device and to provide a steering reaction to the vehicle operator, and a steering motion sensor adapted to sense the steering input motion and transmit a motion signal to the controller. The steering actuator assembly includes means operable to vary the steering reaction provided to the vehicle operator in response to variations in the feedback signal generated by the controller.
The improved steer-by-wire vehicle steering system is characterized by the steering reaction device comprising a housing defining a fluid inlet port and a fluid outlet port, a fluid displacement mechanism, and valving operable to control the flow of fluid from the inlet port through the displacement mechanism to the outlet port in response to movement of the steering input device. The valving defines a neutral condition blocking fluid flow through the displacement mechanism, and includes means biasing the valving toward the neutral condition.
In accordance with another aspect of the invention, an improved steering system is provided of the type including a friction type steering reaction device adapted to receive a manual steering input, the reaction device including a first member and friction reaction means to resist rotation of the first member.
The improved steering system is characterized by the reaction device including a spool member fixed to rotate with the manual steering input, and a sleeve member fixed to rotate with the first member of the reaction device, and at least partially surrounding the spool member. A spring biasing arrangement has a first end seated relative to the spool member and a second end seated relative to the sleeve member. The spool member and the sleeve member are configured to permit a predetermined rotational displacement between the spool member and the sleeve member in opposition to an increasing biasing force exerted by the spring biasing arrangement.